This invention relates to novel improvements and use in a method and apparatus for using photoresistive materials, and more particularly, but not by way of limitation, to provide a switchable electromagnetic interference (EMI) barrier.
Electrical circuitry often must be protected from disruptions caused by EMI entering the system. External EMI energy is an undesired conducted or radiated electrical disturbance that can interfere with the operation of electric equipment. EMI interference describes redistribution of energy in space or time because of reinforcement and cancellation of parts of the disturbance. When the same frequencies are in proximity to each other, exact reinforcement or complete cancellation can occur depending on the phasing of the waves. Slightly different frequencies interfere to produce beats, alternate reinforcements and cancellation that are periodic with time.
Interference is the process whereby two or more waves of the same frequency or wavelength combine to form a wave whose amplitude is the sum of the amplitudes of the interfering waves. If the two waves are of equal amplitude, they can cancel each other out so the resulting amplitude is zero. In optics, this cancellation can occur for particular wavelengths in a situation where white light is a source. The resulting light will appear colored. This phenomenon gives rise to the iridescent colors of beetles' wings and mother-of-pearl, where the substances involved are actually colorless or transparent. Many methods exist using mirrors or prisms to illustrate the interference that can result from different frequencies.
With the development of nuclear explosives, another type of electromagnetic radiation has been observed. Nuclear explosion, and in some circumstances large scale chemical explosions, produce a sharp pulse (large impulse type) of radio frequency (long wave length) electromagnetic radiation. The intense electrical and magnetic fields created by electromagnetic pulse (EMP) energy can damage unprotected electrical and electronic equipment over a wide area. As a result, a demand has appeared for materials that can provide sufficient, or substantial shielding effectiveness against EMP energy threats.
"Smart" materials can be classified as materials that react or take an action to an external stimulus to provide a useful result. Cadmium Sulfide is a well known photoresistor used in lamps and light fixtures around homes and businesses to turn lights on automatically after dusk, and then off again at dawn. In the process of performing this function, the material becomes more or less conductive based on the presence or absence of light.
Photoconductive effects, in which the radiation changes the electrical conductivity of the material upon which it is incident, have been known for many years. There are two types of photoconduction extrinsic and intrinsic. In the intrinsic case, the photoconduction is produced by absorption of light to create a band-to-band transition across the bandgap, where the absorption coefficient is very large because of the large number of available electron states associated with the conduction and valence bands. With the advance of microfabrication technology, photoconductive switches of various configurations have been fabricated in different materials. The addition of light photons to a cadmium sulfide compound results in the freeing up of free electrons which are able to conduct current. As the material becomes more conductive, the inherent ability to block RF energy becomes apparent.
An optical interferometer is based on both two-beam interference and multiple-beam interference of light. Typically these phenomena are extremely powerful tools for metrology and spectroscopy and a wide variety of measurements can be performed. Other types of interferometers exist. Two basic classes exist: division of wavefront and division of amplitude.
Radar-absorbing materials are designed to reduce the reflection of electromagnetic radiation by a conducting surface in the frequency range from approximately 100 MHz to 100 GHz. The level of reduction achieved varies from a few decibels to greater than 50 dB, in percentage terms reducing the reflected energy by up to 99.999%. The performance of any material as a microwave absorber can be calculated from Maxwell's equations if the electrical and magnetic properties are known. However, in the most simple terms, two conditions are necessary to produce absorption. First, the characteristic impedance of the material must match the characteristic impedance of free space so that the electromagnetic energy may enter the material. Second, the material must then attenuate the electromagnetic radiation, which means that it must exhibit either dielectric or magnetic loss, or both.
Microwave-absorbing materials are widely used both within the electronics industry and for defense purposes. Their uses can be classified into three major areas: (1) for test purposes so that accurate measurements can be made on microwave equipment unaffected by spurious reflected signals, such as the anechoic chamber; (2) to improve the performance of any practical microwave system by removing unwanted reflections which can occur if there is any conducting material in the radiation path, and (3) to camouflage a military target by reducing the reflected radar signal.
Despite the theoretical possibility of absorption, in practice, materials have not been found which will give a good impedance match over an appreciable frequency range. It is therefor necessary to adopt specific design methods to manufacture practical absorbing materials.
Two methods have been widely adopted in order to produce such absorbers. The first is to avoid a discrete change of impedance at the material surface by gradually varying the impedance. For example, a thick profiled lossy layer could be used. The removal of the discrete discontinuity at the surface allows the microwave energy to be transmitted into the absorbing medium without reflection. Tapering of the material over distances which are large compared with the wavelength provide this absorption characteristic. Practical absorbers giving greater than 20 dB absorption vary in thickness from about 0.8 inches (2 cm) at 10 GHz and above to six feet (2 m) at 100 MHz and above.
A second technique provides for much thinner absorption layers. These materials consist of lossy layers where the absorption is produced by a destructive interference at the frequency for which the material is electrically a quarter wavelength. The performance is a function of the wavelength frequency, and is tunable from 100 MHz to 100 GHz. In addition to providing a relatively narrow bandwidth frequency performance, it is possible to broaden the bandwidth through a technique of multiple layer absorbers. With two layers of material it is possible to tune one absorber to two different frequencies. By placing these two frequencies appropriately, such as within one octave of each other, a broadband absorber is obtained.
Prior art has shown developments in use of apparatus in the form of seals as one way of providing the necessary shielding. Electrical connectors are illustrated in COOPER et al U.S. Pat. No. 4,330,166, and static housing or gasket seals for equipment cabinets, as illustrated in KEELER U.S. Pat. No. 4,061,413. NEHER U.S. Pat. No. 4,807,891 describes a resilient metal bellows surrounding a static electromagnetic pulse rotary seal.
Existing apparatus and methods only partially solve the problems overcome by the present invention. Finally, current known technology has different purposes than the present invention, not just different applications. One difficulty with the mentioned prior art is that NEHER is applicable to parts moving in relation to each other, whereas the present invention involves static parts. In addition, the other prior art also does not provide for permitting electromagnetic radiation to pass through when required. The connectors or seals are only designed to prevent the transmittal of radiation.